The Upper Usumacinta freshwater ecoregion, FEOW ecoregion 174, is a tropical and subtropical river ecoregion of southern Mexico and Guatemala. It represents the upper and middle parts of the Usumacinta system, especially upland and foothill tributaries draining the highlands of Chiapas and Guatemala before they join the broader lowland Usumacinta and Grijalva-Usumacinta floodplain complex. It should be distinguished from the lower Grijalva - Usumacinta ecoregion, which includes the extensive Tabasco lowlands, floodplain lagoons, swamps, and coastal wetlands.

Geographically, the ecoregion encompasses rivers and tributaries of the upper Usumacinta basin, including systems such as the Lacantún, Jataté, Ixcán, Chixoy/Salinas, Pasión, San Pedro, and related tributaries, depending on the boundary interpretation. In Mexico, it is especially relevant to Chiapas and the Lacandon region, where forested tributaries, karst streams, foothill rivers, and larger channels connect with Guatemala’s upper Usumacinta drainage. The ecoregion lies in one of the most biologically important freshwater corridors of northern Middle America.

The habitats of the Upper Usumacinta ecoregion include clear and turbid rivers, forest streams, foothill tributaries, karstic waters, rocky channels, sandy runs, pools, backwaters, spring-fed streams, lowland transition reaches, floodplain margins, and seasonally connected aquatic habitats. Compared with the lower Grijalva - Usumacinta plain, this ecoregion has a stronger upland, tributary, and forest-river character. Many habitats are influenced by limestone geology, forest cover, seasonal rainfall, and the transition from highland streams to larger tropical rivers.

The fish fauna is highly important for Freshwater Fishes of Mexico because it forms part of the broader Usumacinta freshwater-diversity center. It includes cichlids, poeciliids, characids, catfishes, atherinopsids, gars, gobies, sleepers, eels, mullets, snooks, and other freshwater or freshwater-associated fishes. The fauna varies strongly among forest streams, karstic tributaries, larger rivers, lowland transition reaches, and backwaters. Locality data are essential because tributaries such as the Lacantún, Ixcán, Pasión, San Pedro, Chixoy/Salinas, and smaller forest streams may contain different assemblages.

Poeciliids are one of the defining non-cichlid fish groups of the Upper Usumacinta. Species of Belonesox, Gambusia, Heterandria, Poecilia, Poeciliopsis, Priapella, Xiphophorus, and related genera may occur in different parts of the broader Usumacinta system, depending on drainage, elevation, flow, and habitat. They are especially important in vegetated margins, lowland tributaries, pools, backwaters, springs, and seasonally variable habitats. Some are widespread and ecologically flexible, while others may be localized or tied to particular tributary systems.

Characids and catfishes are also important components of the fauna. Characids occur in streams, rivers, backwaters, and lowland transition habitats, while catfishes occupy a variety of riverine, benthic, cave, and lowland habitats. Heptapterids, pimelodids, ariids in lower or transitional reaches, and other catfish groups add ecological diversity to the basin. Some catfishes are associated with flowing water, others with deeper channels, sandy or muddy bottoms, or structurally complex habitats. These groups help distinguish the Usumacinta from many smaller Mexican coastal drainages.

Peripheral and estuarine-associated fishes become more important downstream, but some may penetrate into the broader Usumacinta system depending on habitat and connectivity. Gars, snooks, eels, gobies, sleepers, mullets, and other fishes may use larger channels, lowland reaches, or connected waters. In the upper ecoregion, the fauna is more strongly riverine and tributary-based than in the lower floodplain, but it still forms part of a large drainage continuum connecting uplands, lowlands, wetlands, and the Gulf coastal plain.

Cichlids are among the most important fishes of the Upper Usumacinta ecoregion. They do not form a lake-type species flock, but they represent a rich assemblage of Middle American river, tributary, karst-stream, lowland-channel, pool, backwater, and floodplain-margin species. Important genera include Chuco, Cribroheros, Kihnichthys, Maskaheros, Mayaheros, Oscura, Parachromis, Paraneetroplus, Petenia, Rocio, Theraps, Thorichthys, Trichromis, Vieja, and Wampaheros, depending on locality and current taxonomy.

Important native cichlids in the broader upper and middle Usumacinta context include Kihnichthys ufermanni, Maskaheros regani, Mayaheros urophthalmus, Oscura heterospila, Parachromis friedrichsthalii, Parachromis multifasciatus, Paraneetroplus gibbiceps, Petenia splendida, Rocio octofasciata, Thorichthys meeki, Trichromis salvini, Vieja bifasciata, Vieja breidohri, Vieja hartwegi, and Vieja melanurus, although each should be checked carefully by drainage and locality. Some species are more characteristic of larger lowland rivers and lagoons, while others occur in tributaries, forest streams, rocky reaches, or karstic habitats.

Taxonomic caution is essential. Many older records use broad Cichlasoma names for fishes now placed in several different genera. Records from the Lacantún, Jataté, Ixcán, Chixoy/Salinas, Pasión, San Pedro, Petén, and Guatemalan tributaries should not be treated as interchangeable without locality evidence. In this ecoregion, a species list without locality can be dangerously neat. Drainage, elevation, habitat, and current taxonomy should always be preserved in FFM records.

Introduced fishes should be treated separately from the native fauna. Tilapiines such as Oreochromis niloticus, Oreochromis mossambicus, and related aquaculture forms may occur in reservoirs, ponds, modified waters, or lowland habitats, but they are not part of the original native fauna. Other introduced fishes, including loricariids, or “plecos,” aquarium releases, stocked fishes, and aquaculture escapees, may also affect native species through competition, predation, habitat alteration, disease, and changes in food webs. Non-native fishes are already documented in parts of the Usumacinta basin and should be considered a major conservation issue.

Conservation priorities include protecting forest streams, karstic tributaries, large river channels, backwaters, riparian forests, springs, floodplain-margin habitats, water quality, and natural flow regimes. Major threats include deforestation, cattle ranching, agriculture, sedimentation, mining, road construction, dams, hydrological alteration, sewage, pollution, invasive fishes, and habitat fragmentation. The Lacandon region and adjacent Guatemalan tributaries are especially important because they retain relatively high-quality forest-river habitats compared with many more modified lowland systems.

This account is based in part on information from Freshwater Ecoregions of the World (FEOW), © WWF/TNC, used as a reference for educational and scientific purposes. See: http://www.feow.org.